Acrocalymma triseptatum Calabon, E.B.G. Jones & K.D. Hyde, in Dong et al., Fungal Diversity [17] (2025)

Index Fungorum number: IF 902548; MycoBank number: MB 902548; Facesoffungi number: FoF 16394; Fig. 1

Etymology – referring to the 3-septate conidia of the fungus.

Holotype – MFLU 24-0268

Saprobic on submerged decaying wood. Sexual morph: undetermined. Asexual morph: Conidiomata 150–170 × 100–115 µm, pycnidial, dark brown to black, solitary, immersed, globose to subglobose, unilocular, glabrous and ostiolate. Ostiole 30–45 µm diam., centrally located. Peridium 11–16.5 µm thick, composed of thick-walled, dark brown cells of textura globulosa in the outer layer, become hyaline cells of textura angularis in the inner layer. Conidiophores reduced to conidiogenous cells or a supporting cell. Conidiogenous cells 11–34 × 2.5–5.1 µm (x̄ = 22 × 4 µm, n = 30), enteroblastic, ampulliform to doliiform, hyaline, smooth-walled. Conidia 17–23 × 3–5.5 µm (x̄ = 19 × 4.1 µm, n = 30), cylindrical with sub-obtuse apex and base, straight, hyaline, aseptate when immature, mostly 3-septate at maturity, constricted at the septum, smooth-walled, small to large guttulate, with a flaring mucoid apical appendage (3–5.5 µm diam.), visible in water mounts.

Culture characteristics – colonies on MEA reaching 30–35 mm diam. at 25 °C after 25 days, flat, dry, rough, margin entire; from above, brown; from below, dark brown; not producing pigmentation in the culture; no sporulation in the cultures.

Material examined – Thailand, Prachuap Khiri Khan Province, on submerged decaying wood, 23 March 2021, M.S. Calabon, HY05 (MFLU 24-0268, holotype); ex-type culture, MFLUCC 24-0318, HY04 (MFLU 24-0267, paratype).

GenBank accession numbers – ITS: PQ285209, LSU: PQ285210.

Notes – the combined ITS, LSU, and SSU phylogenetic tree reveals that Acrocalymma triseptatum MFLUCC 24-0318 clusters within Acrocalymma, sharing a common lineage with A. walkeri (UTHSC DI16-195) and A. vagum (CPC 24225 and CPC 24226) (Fig. 2). However, A. triseptatum can be easily distinguished from A. vagum by its mostly 3-septate, hyaline conidia at maturity, with a flaring mucoid apical appendage. In contrast, A. vagum has 1–3-septate, hyaline to brown conidia at maturity, with dark pigmentation at septa and without any appendages (Farr et al. 1998). Acrocalymma walkeri differs from A. triseptatum in having quite long pycnidial beak, mostly aseptate conidia, with globoid appendages at the ends (Shoemaker et al. 1991). Notably, A. triseptatum represents the second species of Acrocalymma associated with estuarine environments and is the sixth species reported from aquatic habitats (Zhang et al. 2012a; Dong et al. 2020; Boonmee et al. 2021; Calabon et al. 2022, 2023; Manawasinghe et al. 2024). It is noteworthy that A. walkeri (UTHSC DI16-195) clusters between two strains of A. vagum (CPC 24226 and CPC 24225), which is similar to the previous study of Dong et al. (2020). Further investigation into the phylogenetic relationships between the two species is needed in the future.

Figure 1 – Acrocalymma triseptatum (MFLU 24-0268, holotype). a Appearance of immersed conidiomata on wood surface. b Vertical section of conidioma. c Section through the peridium. d–h Conidiogenous cells. i–n Conidia with appendages. Scale bars: a =200 µm, b = 50 µm, c–n = 5 µm.

Figure 2 – Maximum likelihood consensus tree inferred from the combined ITS, LSU, and SSU multiple sequence alignments. Bootstrap support values for maximum likelihood (ML, first value) equal to or greater than 75% and Bayesian posterior probabilities from MCMC analyses (BYPP, second value) equal to or greater than 0.90 are given nearby the nodes. The scale bar indicates expected changes per site. The tree is rooted to Boeremia exigua (CBS 431.74) and B. foveata (CBS 341.67). Ex-type strains are indicated with “T”. The newly generated sequence is indicated in red.